The present trend in telecommunications involves transmitting and switching digital signal streams at higher and higher rates, so as to allow the exchange of increasing amounts of speech, video information and the like, in an integrated network. However electrical signal transmission and processing by electrical systems have speed limits which may be exceeded only by use of optical technologies. Advanced telecommunications networks thus have node interconnections implemented by wideband and low-attenuation optical fibers, but wherein optical-signal switching means still make use of basically electrical technologies.
Optical technology should, however, also be used in switching systems, not to hinder the potential presented by the optical fiber transmission network in terms of bandwidth. This would also help avoid repeated optical-to-electrical signal conversions and vice versa, flow splitting and recombinations, and generally all of this tricks used to overcome limitations inherent in electrical systems.
Optical and electro-optical devices, capable of acting as space switches, modulators, filters, amplifiers, and the like, which can be advantageously used in optical signal switches, have been investigated and developed. Communication systems have been developed wherein the number of functions implemented by electrical devices has been increasingly reduced. In the near future, entirely optical elements might be used also for control functions.
At the same time, fast packet switching systems, which prove to be more and more convenient to resolving the problem of integrating the switching of different-characteristic streams carrying information relevant to speech, video and data signals, are being investigated. These systems, in fact, allow resources to be exploited in an optimized way, since they occupy the network only in proportion to actual requirements, instant by instant, without requiring the structures to be strictly allotted to a connection during idle periods of the source, and which thus would remain unused.
Besides, structure and technology reliability allows heavy information processing at each network node, which would imply delays and delay variations not to be neglected in speech or video signal transmission. Thus integration of transmission and switching of such signals with data signals is possible.
Various switching systems have been described in the literature. These include systems wherein the electrical connection network consists of an optical connection network, generally controlled by electrical devices. One of them is the packet switching system described in the paper entitled: "Optical Technology Application to Fast Packet Switching" by P. Cinato and A. De Bosio, published in the Proceedings of the Conference "Topical Meeting on Photonic Switching", Mar. 1-3, 1989, Salt Lake City, U.S.A. In this system a binary optical connection network system is configured by an electrical control network which is also binary and self-routing, upon passage of electrical packets containing the routing information. At the end, the information field proper is sent into the optical network.
Another packet switch system is described in the paper entitled "Demonstration of Fast Wavelength Tuning for a High Performance Packet Switch", by M. S. Goodman et al issued in the Proceedings of ECOC 88 Conference Sept. 11-15, 1988, Brighton, UK, pages 255-258. This system uses two superimposed networks, one for routing control packets, the other for the information packets proper. At the input the optical packets are converted into electrical signals in order to be temporarily stored in memory units; then they are switched by allotting them a suitable frequency, chosen on the basis of the output towards which they are directed.
A further system has been described in the paper entitled "A Photonic Knockout Switch for High-Speed Packet Networks", by Kay Y. Eng, published in the Journal on Selected Areas in Communications, August 88, Vol. 6, n. 7, pages 1107-1116. In this case, the incoming electrical streams are converted into optical signals and sent to the switch by associating each input with a specific optical wavelength. Here all the streams are summed in a star coupler and sent towards means for selecting the wavelength to be sent to the corresponding output. A parallel network performs all the decisions inherent in the contention resolution by using an algorithm envisaging the possible packet loss. The processing result of this control network consists in commands for means selecting the flows at different wavelengths, which can thus operate to forward the streams to their corresponding outputs.
These systems either use the optical carrier to transport high-bit rate information or different wavelengths to facilitate the space switching inside the switch, but do not exploit the possibilities offered by the wideband of the optical fibers for the simultaneous transport of a plurality of channels. In other words, they use a packet and space switching, but do not use a frequency switching proper. Besides, temporarily packet memorization is performed in memories of the electronic type, after converting them from optical to electric signals.
From the switch control standpoint, both the investigated systems require a time phase for a knock-out tournament for the resolution of the conflicts at the outputs, when different packets are addressed to the same destination. That entails the necessity of substracting a time interval from the usual packet transit time, which solely depends on the bit rate of the handled flow. As a consequence, switching has to be carried out at higher rate than that required by the link bit rate.